Telephone system



TELEPHONE SYSTEM Filed Jan. 17, 1934 2 Sheets-Sheet l I lNl ENTOR R. H. F LOWE RS By %4 Q;

A 7'7ORNE V March 12, 1935.

R. H. FLOWERS 1,993,833

TELEPHONE SYSTEM Filed Ja n. 1'7, 1934 /N VEN TQR RH. FLOWERS ATTORNEY 2 Sheets-Shet 2 Patented Mar. 12, 1935 UNITED STATES TELEPHONE SYSTEM Rea Hollway Flowers, Indianapolis, Ind., assignor QtoVAmerican Telephone and Telegraph Company, a corporation of New York;

Application January 17, 1934, Serialfio. 707,012

' 7 Claims. (o1.-'179 1s This invention relates to telephone systems and more particularly to systems employing plunger type line switches.

The object of this invention is-to improve the 5 operation'of'plunger type line switches and to render their operation more reliable by eliminating the possibility of double. connections.

The construction, operation and utility of plunger type line switches are too well known to require a detailed description here. It is also well known that systems employing these switches aresubject to trouble from double connections caused by two or more switches being operated simultaneously- Such a seizure'of'one trunk'by more than one line switch occurs when a plurality of calls are simultaneously initiated on lines associated with the same masterswitch. 'Since the battery is disconnected from the pulldown windings of the magnets of all line switches of a'group duringv the time that the shaft of the master switch is in motion, the line switches are prevented from operating to extend calls which have originated while the master switch is operating, with the result that double connections may also occur in case two or more calls are waiting when the operation of the master switch ceases and battery is again connected to the pull down windings of the magnets of the associated line switches.

The present invention provides an arrangement for controlling the'battery connection for a group-of plunger type line switches, in which a relay bridge is connected across two normally balanced current paths, the simultaneous initia-- tion of a plurality of calls in'one line group being efiective to operate the relay due to-an unbalance thereby created between the two paths. The operation of the relay increases the'rcsistance in the battery connection sufiiciently to prevent the complete operation of any of the line switches; The current is suificient, however, to operate the bridge cut-off armature of the switches'in question and thereby cause release-of the associated line relays. When all but a single line relay has released, the'balance between the two current paths is restored, thereby causingthe release of the bridge relay and the consequent closure of the battery connection to cause the complete operation of the associated line switch.

The invention further, provides an arrangement in which the ground connection for operating a group of line switches is controlled by a relay common to the switches and by chain contacts on a relay for each of theswitches, the common relay being operated when twoor more switches are seized simultaneously to prevent seizure of the same trunk by two or more of the switches. V

The invention will Ice-explained by reference to the accompanying drawings, inwhich Fig. 1 and Fig. Ztaken together, and with Fig. 1 at the left of Fig. 2, show one of .the forms in which" the in vention may be practiced;

t The organization illustrated in the drawings,

comprises-as much of a complete telephone system as is necessary for a clear understanding of the manner in whichthe invention is organized and operates. There are shown in the drawings two calling lines extending from stations A a C1 A1 to a central officaone called line extending rom the central ofilce to station Baand connecting circuits at the central office, by means of which the telephone lines may be joined for conversation.

The connecting circuits include individual line switches PLS1 and. PLS '75, secondary line switohes SLS1 and SLS-,-25 together with their master switches PMS and SMS allof which may be of the type described inPatent No. 1,586,530,

granted June 1, 1926 to J. H. Homrighous The connecting circuits iurther include a selector D and connector Ewhich may be of thetype shown in Patent No. 1,379,068 granted May 24:, 19'2 1 to J. Wicks; The particular parts of this-system to whichthe inventionis directed are shown at C andCl.

- Having given a general description of the apparatus involved in the system, the invention will be most readily under'stoodfrom a detailed description of the operationsinvolved in setting up aconnection P Upon the removal of the receiver at the calling station'A'preparatory to making a call, relay I 101 of'the primary line switch PLS1 is operated-overthe calling line circuit. Relay 101 in 115, winding 102 of switch PLS-l, front contactof relay 101, conductor 140, through the low resistance left winding of relay 126 to ground. Re-' lay 105- is operated by the current in this circuit to complete a Wheatstone'bridge as hereinafter described. I In addition to extendingthe calling 7 line over conductors 111 and'112-to the secondary line switch SLS -l, a circuit for operating the secondary line switch is closed from ground at contact 121 of springs 138, thence overconductor 113, through resistance 222 at 01 in Fig 2, winding of relay 204, conductor. 213, through pulldown winding 2'02andnormally closed contact of secondary line switch: S LS hthrough the winding of relay 201, conductor 214, left-hand armature and front contact of relay 207, contact on relay 21G, thence over conductor 215, right back contact relay 212, conductor 216 to battery through the left winding of relay 211. The pulldown winding 202 in the path just traced does not receive sufficient current at this time to operate line switch plunger 241 due to the resistance 222.

which according to this invention has beenin-v serted for preventing double .ccnnections. Relay 201, however, operates toestablish at its front contact a temporary holding circuit for the PLS-1 switch traced from battery through winding 103, contact 122 of springs 138, conductor 114, contact of relay 201, conductor 213, winding of relay 204, resistance 222, over conductor 113 to ground at contact 121 of springs 138.

The circuit for operating the secondary line I switch SL S1 is 'dependenton the operation of relay 120.4 of the apparatus at 01 as will hereinafter be explained. Assuming for the present that pull-downwinding 202 operates plunger 241 to extend the calling'line "to selector 13, contact 239 opens the circuit through the winding of relay 201 which after .aninterval releases and removes that portion of the holding circuit for winding 103 of switch PLS1: traced from the armature ofrelay 201 over conductor 213 to ground at contact 121 of switch P-LS1. Line switch PLS1 does not however release because in the meantime the operation of plunger 241 of secondary switch Slii'S-l closes contacts 240 and extends the holding circuit for winding 103 of switchPLS-l to conductor 2 19 of selector D. Ground is now returned over conductor 219, before relay 201 releases by'-selector 'D in the well known manner. When therefore the subscriber .dialsthe wanted number, the selector Dandconnector E function to extend the calling line to the called subscriber B in the'usual' manner described in above mentioned Patent No. 1,379,068 to Wicks. The holding circuit for winding .103. of the primary line switch PL'S1 is new extended to the connector E and remains under control of thecalling subscriber. 'The holding circuit for the, secondary line switch SLS-1 includes windings 203 and 202,1oontact 220, conductor 213, winding of relay 204, resistance 222., .conductor'113 tog-round at contact '121 of primary line .switch PLS-1'.

- Referring now to the operation of the primary line switch PLS-l it will be seen that battery for energizing the pull-down windings 102 of all primary line switches associated with the primary master switch PMS, which in previous systems of. this kind it has beencustomary to supply directly from the master switch, is accord,- ing to this invention supplied over conductor from the master switch PMS under control of relays .105 and.106 at C. Operation of the primary master switch PMS does not differ essentially'from the usual manner of operating such switches but the following description will howeveraid in obtaining a clearer understanding of. the arrangement for preventing double connectionsat primary line'switches.

'Theoperation of primary line switch plunger 141 closescontact 121 of springs 138 and connects ground over-conductor .118 to master switch bank contact 123, corresponding to the preselected trunk,rthence through master switch wiper 134 conductor 119 and winding of relay 128 to battery. Relay 128 operates and at contact 124 closes a circuit from battery over conductor 120, through the winding of relay 104, and through the low resistance left hand winding of relay 126 to ground. Relay 104 operates and opens the main battery supply over conductor to the pull-down windings of all switches associated this particular primary master switch. Line switch PLS 1 does notrelease, however, because of having established a holding circuit for itself over lead .114 as previously described. In the circuit traced above for opening the battery feed to all line switches of a group it should be observed that the operation of relay 128 also disengages the locking arm of the primary master switch PMS allowing the master switch spring 137 to move the line switch shaft 116 together with its idle line switches and master switch wiper 134 to the next idle trunk in the usual well known manner. The master switch passes over trunks that are busy since the master switch bank contacts of busy trunks will be grounded from the line switch engaged therewith over a circuit similar to the one described above for operating relay 127 when switch PLS1 operated. The masterswitch stops on an idle trunk due to the absence of ground on the master switch bank contact. Relays 127 and 128 then release in turn to permit the locking arm 135 to reengage the locking segment 136. Relay 128 releasing also releases relay104 which again connects battery'to feed conductor 125.

The aforementioned operation of relay 105, (due to the operation of line relay 101), com pletes a Wheatstone bridge comprising resistance 107 as one branch, resistance 108 equal to resistance 107 as another branch, and resistance 103 as a thirdbranch. The fourth branch of the bridge includes the winding of relay 105, conductor. 115 and the pull-down winding 102 of switch PLS1 in parallel with the pull-down winding of any other switch the line relay of which is operated simultaneously with relay 101 of switch PLS-1. Resistance 109 is approximatelyequal to the sunset the resistances of the will be low enough to unbalance the bridge and:

a sufficiently large potential difference will then exist across the winding of relay 106 to cause its operation.

lease of eachof these line relays opens the cir' cuit through the associatedpull-downwinding and the release of relay 105 destroys the bridge and causes the release of relay 106.

Line relay 101 and relays 105 and 106 again operate and again restore to normal as just described and would continue. in this way' to pre- If relay 106 operates due to unbalanced condition created by the simultaneous operation of aplurality of line relays in ondary line switches are prevented.

vent the calling lines fromjbein'gconnectedto the same trunk. It is wellknownthat the time characteristics for the operation and'release of such circuits differ among themselves to s'uch'an extent that after one. or. several operations they get out of step. Assuming then for example that relay. 101 remains operated after the other line relays have released, the balance. of the bridge is restored causing relay 106 to'be deenergized. The resistance 110 is thus shunted out to cause the primary line switch plunger 141 to be operated'and connectthe calling subscriber A to the idle trunk. Another callingsubscriber. such as A1 is prevented from seizing'the sametrunkeven if the associated line relay operates immediately following the operation of'plunger141 oi? switch PLS1 because the connection of: anotherpulldown winding in parallel with the. pull-down winding of switch PLS1 causes-thereoperation of relay 106 to introduce resistance 110 in the battery supply circuit. Plunger 141 of switch PLS1.that has already operated. continues to hold, in spite of the increased resistance in the battery supply circuit, but the pull-down winding of the other switch doesnot receive sufficient-:- current to operate its plunger.) Thus it "will be.

trunk after being held up on account of removal of battery from the pull-down windings by the master switch in. its search for an idle trunk.

Furthermore, it will be evident that this same.

circuit arrangement. is :similarlyeffec'tive in those cases where secondary line switches'are not installed and the trunks lead directly to selectors. l Y V i A description will now be given of the circuit arrangement by which double connections at se'c' It is of course understood that every trunk from a primary line switch to a secondary line switchis equipped with a resistance and relay such as is shown at 222 and 204 for one trunk and at 223 and 205 for another trunk- Relays 224 and 225 are common to all the relays such as 204 and205 that are associated with trunks tothe same group of. secondary line switches and control a chain circuit through these individual relays As hereinbefore described, on a call from station A to station B, the operation of the secondary line switch SLS1 in a'path'including resist--' ance 222 and relay 204 was stated tolbedependent on the operationof relay 204. The resistance 222 is of such a value as willp-revent the secondary'line switch from plunging but will permit relay. 204 to operate. The operation of relay 204 then closes a circuit for operating thesecondary line switch; this circuit is traced from ground at the back contact of relay 224, conductor 242, back contact of relay 205, and 'back contacts of intermediate relays similar to relay 205, lower front contact and winding of relay 204; conductor 213, pull-down winding 202 and back contact of switch SLS-l, winding of line relay 201,

over conductor 214, throughback contacts of relays 207 and 2100f the "master switch'SMS,over" fore, by way of example, two primary line switches in different groups o'f; primary line switches connect simultaneously to-line switches in the same secondary group as SLS-1 and SLS25, relays 204 and 205 both operate. 7. The

operation of relay 205 closes a circuit from ground at the back contact of relay224, over ter switch SMS as 'hereinbefore traced. Since relay 205 is the first relay in the chain circuit from ground at the backcontact of relay 224, the operation of relay.205 disconnects this ground from all of the succeeding relays in the chain so that relay 205 alone is efiective to cause the energization of the pull-down winding .of the associated line switch. The operation of relays 205.

and 204 also completes a circuit for operating relay 224; this circuit is tracedirom battery through the winding of relay 224,1ower front contact of relay 205, through the upper back contacts ofintermediate relays similar to "relay 205, through the upperifront contact of relay 204, through resistance 222, over conductor 113'," to ground at contact 121 of springs 138 of the primary line switch PLS-l. Relay 224 is slower in operating than the pull-down'winding of a secondary line switch so that switch SLS" is'at least partially operated before relay 224 opens its back contact; and the operation. of line switch SLS-25 is then completed by the energization of both of its windings in series, this cir-, cuit being traced from battery through winding 253, the front contact and winding 252 of switch SLS25 in series, conductor 243, winding of relay 205, resistance 223, conductor 229 to ground at the primary line switch by which this trunk was seized.

The operation of switch SLS25 extends the 7 connection from trunkconductors '226 and 227 over the trunk opposite whose terminal springs the plunger of switch SLS-25 is at that time standing to a selector similarto D. The opera-' tion of switch'SLS+25 also opens the circuit through its linerelay 251 in the well known man-.

ner but the, line relay being slow to release holds operated fora short period andpermits the ground thatis returned from the selector over the conductor corresponding to conductor 219 to mo- 'mentarily, pass through front contact of relay 201 switch'selects another idle trunk and reconnects battery over conductor 214, the switch'SLS--1 operates to extend the trunk conductors 111 and 112'to an idle selector.

In the case of the two simultaneous calls just.

described, it was assumed that relay 224 does not remain operated long enough for the slow operate relay 225 to operate. When however during the busy period there-is a scarcity oftrunks to selectors and it is necessary for the master switch SMS- to continue its search for an idle trunk, relay 224 may remain operated long enoughto cause the operation of relay 225. In this case relay 225 restores ground .to the chain circuit throughuthe individual, relays .205 and 204. The time interval elapsing before relay 225 operates is sufilciently long to, permit the master switch SMS to function and open the battery supply over conductor 214 at the back contact of relay 210. In this way the secondary line switch connected to the individual relay nearest the common relays 224 and 225 will be in a position to operate as soon as the master switch restores main battery to the secondary line switches.

What is claimed is:

1. In atelephone system, a group of lines, an

electromagnetically operated plunger type line switch for each of said lines, a battery and a corn ductor for connecting said battery to the operating magnet or" each of said switches, a bridge circuit comprising. said conductor completed in response to the initiation of a call over any of said lines, said bridge being unbalanced when calls are initiated simultaneously over a plurality of said lines, and means responsive to said bridge being unbalanced for preventing the operation of any of said switches.

2. In a telephone system, a group of lines, an electromagnetically operated line switch for each line, circuits for operating the magnets of said switches, a first group relay the winding of which is included in each of said operating circuits, a shunt circuit closed by the operation of said first.

group relay, and a second group relay for preventing the simultaneous operation of more than one of said line switches, the winding of said second group relay being bridged acrosssaid shunt cir-' cuit and said operating circuits.

3. In a telephone system, a group of plunger type line switches each switch comprising a pulldown winding, a line relay foreach switch, a first group relay, a second group relay, an operating path for the pull-down winding of each of saidswitches including a front contact of the type line switches, a-relay individual to each of said switches, a relay common to said switchesfor preventing the simultaneous operation of more than one of the switches, circuits for operating the magnets of said switches, each operating circuit being controlled by the associated one of said individual relays and by said common relay, and a circuit closed only when a plurality of said individual relays are operated for operating said common relay.

5. In a telephone system, a group of plunger type line switches, a relay individual to each of said switches for operation upon seizure of the associated switch, a group of trunks connected to the terminal springs of each of said switches, a master switch for positioning the plungers of idle line switches in operative relation to an idle one of said trunks, means responsive to the operation of one of said idle line switches for causing the operation of said master switch to advance the.

plungers of the remaining idle switches to a posi-. tion opposite another idle trunk, circuits for operating the magnets of said switches, each of said operating circuits being controlled by the associated one of said individual relays, a control relay common to said switches for controlling all of said operating circuits, a circuit closed by the simultaneous operation of aplurality of said individual relays for operating said control relay to disable the operating circuits of all but one of said switches, and another relay operatively responsive to the operation of said control relay for rendering said operating circuits operatively effective While said control relay is operated,said other relay being slow in operating to allow the selection of another idle trunk by the operation of said master switch while said operating circuits are disabled.

6. In a telephone system, a plurality of lines, a line relay and a line switch for each line, operating circuits one for each of the line relays, operating magnetsone for each of the line switches, a battery for use in operating said magnets, a first relay common to said switches, operating circuits one for each of said magnets, the operation of any one ofsaid line relays being effective'to close the operating circuit for the magnet of the associated lineswitch, said battery and the winding of said first grouprelay being included in each of the operating circuits of said magnets, a first resistance common to said'line switches and approximately equal to the resistance of the operating winding-of each ofsaid magnets, a circuit including said battery and resistance closed by the operation of. said first relay in response to the operation of anyone of'said line relays, a second relay common to said line switches, and asecond resistance common to said line switches connected in series with said battery and said first relay and normally shore-circuited by contactsof said second relay, the operation'of only one of said line relays at a time being ineffective to cause the operationof said second relay andthe operation of a plurality of said line relays at one-time being effective to cause the operation of said second relay.

7. In artelephone system, a plurality of lines, a line relay and a line switch for each line, operating circuits one for each of the line relays, operating magnets one for .each of the line switches, .a battery for use in. operating said magnets, a first relay common to said switches, operating circuits one for each of said magnets, the operation of any one of said line relays being eifective -to close the operating circuit for the magnet of the associated line switch, said battery and the winding of said first group relay being included in each of the operating circuits of said magnets, a first resistance common to said line switches and approximately equal to the resistance of the operating winding of each of said magnets, a circuit including said battery and resistance closed by the operation of said first relay in response to the operation of anyone of said line relays, a second relay common to said line switches, and a second resistance common to said line switches connected in series with said battery and said first relay and normally short-circuited by contacts of said second relay, the winding of said second relay being normally short-circuited by contacts of said first relay, the operation of only one of said line relays at a time being'ineifective to cause the operation of said second relay and the operation of a plurality of said line relays at one time being effective to cause the operation of'said second relay and thereby prevent the operation of any of said magnets by opening the short circuit across said second resistance.

1 REX HOLLWAY FLOWERS. 

